The present invention generally relates to magnesium-based casting alloys having improved elevated temperature performance and more particularly relates to magnesium-aluminum-strontium alloys having good salt-spray corrosion resistance and good creep resistance, tensile yield strength and bolt-load retention, particularly at elevated temperatures of at least 150xc2x0 C. The present invention also relates to a method of making an oxidation-resistant magnesium alloy melt and to a method of making a magnesium-based alloy casting from such an oxidation-resistant alloy melt. The present invention further relates to the oxidation-resistant alloy melt and alloy casting prepared by such methods.
Magnesium-based alloys have been widely used as cast parts in the aerospace and automotive industries and are mainly based on the following four systems:
Mgxe2x80x94Al system (i.e., AM20, AM50, AM60);
Mgxe2x80x94Alxe2x80x94Zn system (i.e., AZ91D);
Mgxe2x80x94Alxe2x80x94Si system (i.e., AS21, AS41); and
Mgxe2x80x94Alxe2x80x94Rare Earth system (i.e., AE41, AE42).
Magnesium-based alloy cast parts can be produced by conventional casting methods which include diecasting, sand casting, permanent and semi-permanent mold casting, plaster-mold casting and investment casting.
These materials demonstrate a number of particularly advantageous properties that have prompted an increased demand for magnesium-based alloy cast parts in the automotive industry. These properties include low density, high strength-to-weight ratio, good castability, easy machineability and good damping characteristics.
AM and AZ alloys, however, are limited to low-temperature applications where they are known to lose their creep resistance at temperatures above 140xc2x0 C. AS and AE alloys, while developed for higher temperature applications, offer only a small improvement in creep resistance and/or are expensive.
It is therefore an object of the present invention to provide relatively low cost magnesium-based alloys with improved elevated-temperature performance.
It is a more particular object to provide relatively low cost magnesium-aluminum-strontium alloys with good creep resistance, tensile yield strength and bolt-load retention, particularly at elevated temperatures of at least 150xc2x0 C., and good salt-spray corrosion resistance.
It is a further object to provide oxidation-resistant alloy melts and magnesium-based alloy castings, prepared therefrom.
The present invention therefore provides a magnesium-based casting alloy comprising, in weight percent, 2 to 9% aluminum and 0.5 to 7% strontium with the balance being magnesium except for impurities commonly found in magnesium alloys.
The present invention also provides a method of making an oxidation-resistant alloy melt, and the alloy melt prepared by such a method. The alloy melt comprises magnesium as a primary alloying metal, and aluminum and strontium as secondary alloying metals, while the inventive method comprises: melting the alloying metals under an atmosphere of an inert gas selected from a mixture of carbon dioxide and sulfur fluoride gas, a mixture of nitrogen and sulfur dioxide gas, and combinations thereof.
The present invention further provides a method of making a magnesium-based alloy casting from the above-identified alloy melt, and the alloy casting prepared by such a method.
The foregoing and other features and advantages of the present invention will become more apparent from the following description and accompanying drawings.